I am buying a commercial property in north New Jersey. Now I am going through the process getting the loan approval from the bank. I was required by the bank to do an environmental assessment. Among other things, one major concern is that the property had been used as a gas station about 80 years ago. There were no records showing what happened to the underground storage tanks for the fuel. If they are still there and were not properly abandoned, they might pose serious environmental concerns. The property had gone through a lot of changes and nobody knows where the tanks could be. Now the property looks very nice with the new landscaping and the pavement, I don’t want to dig everywhere to find it out. Is there any way to find them without digging? I heard there are some cool techniques for underground storage tank detection.

I used a couple of consulting companies to deal with the similar problems. They were helpful but the results were not as good as I would like them to be. Their technicians generally use metal detectors and the ground penetrating radar. They used spray paint to mark some areas, called "anomaly areas", on the ground. Underground metals (i.e., steel) were detected in these areas. Unfortunately, they couldn’t tell whether there were underground storage tanks. They said the investigation depth of the ground penetration radar was limited on these sites. We did excavation eventually. What we found was just some concrete debris.

The ground penetrating radar doesn’t work on electrically conductive host materials. Clay is a very common problem for the ground penetrating radar. If the radar doesn’t work well, you might have to rely on metal detectors or magnetometers. A magnetometer passively measures the earth magnetic field and could suffer greatly from noises in a city environment. Metal detectors are used more often.

A successful search for underground storage tanks involves a metal detector suitable for the site, an experienced operator, and luck. There are many metal detectors; they have different sensitivity, different investigation depth and different reactions to the noises. Very old underground storage tanks, such as gasoline and kerosene tanks, should be made of steel. Their capacity generally should not be less than 1000 gallon. The majority of them were buried less than 5 feet deep (from the top of the tank to the ground surface), although there are tanks buried much deeper. To avoid unnecessary noises, you should use a less sensitive metal detector; those used for searching coins are not good choices. If the metal detector is most sensitive in the depths where tanks could be buried (i.e., 2 to 5 feet), that would be ideal. The other important factor is the experience of the operator. Experienced operators sometimes could tell the responses of the metal detector are from large or small, deep or shallow, aboveground or underground metals. But still, sometimes there is just no way to tell whether the responses are from underground storage tanks or not.

No real progress after dealing with several banks back and forth. One bank finally recommended a geophysical consulting company do a survey and I had to pay for it. Fortunately, they are not as expensive as I thought. The guy just spent a couple of hours over there and did not find any tanks. In the report, it basically says they did not find any geophysical evidences of underground storage tanks. The bank was satisfied and I eventually can get the a loan with a good rate! Thanks a million!

If I am the buyer or the bank, I will probably be hesitate to proceed with the transaction without any investigations. Sometimes locations of the buried tanks can be found through the their associated pipes. There are a lot of ways to locate them, but it's better to find a experienced guy to do it for you.

Sometimes there are no traces at all about the past USTs and you just have to do a blind searching.Below are examples of two tanks detected and marked with the yellow paint through the GPR survey. There were no fill-ports, no vents or anything related to tanks visible nearby or in the basement.

Simon wrote:Sometimes there are no traces at all about the past USTs and you just have to do a blind searching.Below are examples of two tanks detected and marked with the yellow paint through the GPR survey. There were no fill-ports, no vents or anything related to tanks visible nearby or in the basement.

A lot of common underground storage tanks for either residential or commercial property, such as home heating oil tanks, farm fuel oil storage tanks, gasoline storage tanks and diesel storage tanks, are generally buried from 2 to 5 feet deep. Of course, there are tanks buried much deeper; it may not be rare, but I would say it’s not common. There could be other custom made tanks such as chemical tanks and their burial depths vary depending on their specific requirements.

The existence of the tanks could be unknown until contaminants were found in the soil or underground water nearby because of the leakage. If the tank abandonment was not properly done before, tank removal could be part of the remediation process, but first you need to find them. Simple metal detectors were used a lot, but there were just too many false alarms. Other tools such as the GPR and more advanced electromagnetic sensors should be used to get a better success rate.

Environmental consulting companies and their consultants generally don’t worry too much about fiberglass tanks. They are pretty new like those double wall stainless steel tanks. There should be very good records if they were installed. Just for the record, the GPR can be used for detecting underground fiberglass tanks.

Is it possible to detect USTs underneath a crawl space with GPR? Only the top of the crawl space is accessible. Can the GPR survey be done over the crawl space for USTs? I was told that the void space could be a problem for the GPR because of the "ringing effect", and I don't even know what that means.

A crawl space could cause two problems for the GPR. The GPR for UST searching is usually "ground-coupled", meaning that it has better performance when the antenna is close to the ground for the EM waves being "coupled" into the subsurface. The ringing effect refers to the multi-reflections at the air-soil interface, which is usually very strong and tend to cover the reflections for interested targets. I don't know what's on top of and what's the height of your crawl space, they all could affect the GPR performance. Still, in some cases, you can find USTs beneath or inside a void space with GPR. Here is a GPR data from scanning on top of a basement containing several tanks.

Simon, it looks like you have a lot of experiences on GPR surveys. I don't know whether you are interested, but we have an ongoing project need a GPR survey. Please check your private message for details. Thanks!

EM mapping with Geonics EM61 and EM31 would be a easier method than GPR to locate buried steel tanks. Although you may get noises from surrounding metallic objects, steel tank can give very strong signals and they are very hard to miss. EM mapping is especially good at large sites containing a lot of utilities, small buried metallic junk and building foundations. The signals from buried steel tanks would be different from "noises" generated by these non-target metallic objects in amplitudes, spacial scales and patterns. Some of the USTs can even be detectable underneath reinforced concrete. I strongly recommend doing both EM mapping and GPR surveys for searching for USTs.

I agree that EM mapping is a good method for locating USTs. However, in areas with very limited open spaces or surrounding metals, EM mapping is simply not effective. The signal transmitting and receiving of GPR are very directional (to and from the subsurface directly under the GPR) while EM instruments are prone to get noises originated from objects on or above the ground. Searching USTs such as oil tanks around buildings, between walls or in basements sometimes is really not suitable for the EM method.

I understand what you are saying. Both methods have advantages and limitations, that's why I recommend to use both methods whenever possible. The main problem for GPR is the limited investigation depth while EM can get affected by nearby metals. But EM mapping is not as bad as it's thought to be and it can be used in urban environment if the survey is carefully designed. The EM survey can be done as close as 5' to the building with steel walls and around other obstacles, as shown in the following map where an UST was located with high EM61 readings:

I like this EM mapping idea. GPR penetration depth limitation is a very serious problem. The UST searching surveys often are not conclusive because of this. I am tired of hearing "we are not sure because the GPR penetration depth is limited" from GPR operators. I know it's not their fault, but this EM mapping seems to be a very good alternative when GPR doesn't work well.

As I said in a previous post about EM mapping, EM mapping usually can not give conclusive results because it's a 2D tool without much information in the depth direction. I agree that EM mapping is good method for UST searching, but it's generally not cost effective for small projects in urban environments